Thermal management in semiconductor devices and circuits is a critical design element in any manufacturable and cost-effective electronic and optoelectronic product, such as light generation and electrical signal amplification. The goal of efficient thermal design is to lower the operating temperature of such electronic or optoelectronic devices while maximizing performance (power and speed) and reliability. Examples of such devices are microwave transistors, light-emitting diodes and lasers. Until recently, these devices have been manufactured from silicon, gallium arsenide (GaAs) and indium phosphide (InP). In recent years, gallium nitride (GaN), aluminum nitride (AlN) and other wide-gap semiconductors have surfaced as new choices for both power electronics and visible-light generating optoelectronics. For example, gallium nitride is a wide-gap semiconductor that is used today for visible light-emitting diodes and lasers and for high-power microwave transistors.
The gallium nitride material system gives rise to microwave transistors with high-electron mobility (necessary for high-speed operation), high breakdown voltage (necessary for high power), and thermal conductivity that is greater than GaAs, InP, or silicon, and thus suitable for use in high power applications. GaN is also manufactured at temperatures closer to those used in silicon processing (˜1000° C.). Recently, growth of GaN on silicon has been demonstrated and investigated.
The most investigated gallium-nitride high-power transistor structure is that of the high-electron mobility transistor (HEMT), illustrated in FIG. 1. This transistor comprises a substrate 102 on top of which a layered structure 101 is grown. The layered structure 101 comprises GaN 104 and AlGaN 105 layers on top of which electrical contacts 110, 111, and 107 are deposited. These contacts serve in the operation of the HEMT. Since GaN is a single crystal with a lattice constant that is different from the substrate, it is often necessary to grow several layers to accommodate for the lattice constant change and absorb the dislocations. These layers are collectively referred to as the buffer layer 103, and typically comprise AlN or a combination of GaN and AlN. In the GaN layers 104, close to the semiconductor junction between AlGaN 105 and GaN 104, a layer referred to as the two-dimensional electron gas (2DEG) 108 is formed. Its formation is both an electrostatic and quantum-mechanical phenomenon. The electrons in this thin layer have very high mobility and carry current from the source 110 to the drain 107. This current path is commonly referred to as the channel. The density of the electrons in the channel determines the resistance between the source and the drain and is controlled with the voltage on the gate terminal 111. Finally, using a small voltage applied to the gate terminal 111 one can control very large currents in the channel 108—this is the fundamental requirement for current and power amplification in electronic devices.
Because GaN devices offer high current density and high voltage operation, they exhibit larger total power losses due to parasitic resistances and the inefficiency inherent to the amplification process. Most of heat dissipation in the exemplary device shown in FIG. 1 occurs along the channel 108 and underneath the contacts (source 110 and drain 107). Efficient removal of this heat is essential to making practical GaN HEMTs. However, prior art GaN devices have used substrates that have drawbacks negatively impacting device microwave and/or thermal performance or price. Examples of such substrates are silicon, sapphire and silicon carbide.
Gallium nitride devices have also been investigated for light-emitting diodes for solid-state illumination as well as for medical and environmental laser applications. In all of these applications, heat removal is typically accomplished by placing the electronic device, optical device or integrated circuit as close as possible to a heat sink. A heat sink is a substance or device for the absorption or dissipation of unwanted heat (as from a process or an electronic device). Most often, the heat sinks are copper blocks attached to a water-cooling system, aluminum fins, or a micro-channel cooler. Diamond heat sinks are being actively investigated because of the superior thermal properties of diamond. However, because of the material and process temperature incompatibility, only bonding or die attach methods have been investigated. Conventional heat removal systems for transistors and light-emitting devices (based on bonding and attaching devices to heat sinks) are typically large in comparison with the heat source in the electronic chip or individual device and offer limited thermal performance improvement.
There has therefore been a need for devices, systems and structures that can combine the thermal and other advantages of diamond substrates with wide-gap semiconductors, particularly gallium nitride, aluminum nitride or similar films, and for methods for manufacturing the same.